Use rdjpgxmp to extract the XMP data from a JPEG/JFIF file, use xmptool
to query or update that XMP data, then use wrjpgxmp to put the XMP
data back, e.g.: rdjpgxmp file.jpg | xmptool -w 'http...property'
'value' | wrjpgxmp file.jpg >newfile.jpg. The jpegxmp script is
a wrapper around these to make the most common operations easier.